Ion mobility spectrometry (IMS) is an important technique for the detection of narcotics, explosives, and chemical warfare agents because of its high sensitivity and amenability to miniaturization for field-portable applications. In IMS, gas-phase ions migrate in a drift tube in the presence of a constant electric field. Ions are separated by differences in their drift velocities. For electric field strengths that are relatively low, an ion's drift velocity depends on the applied electric field strength and the mobility, K, which is independent of the applied electric field and experimentally determined. The ions travel through a bath gas at a sufficiently high pressure that allows the ions to rapidly achieve a constant velocity when driven by the force of a constant electric field, which contrasts to migration in a mass spectrometer where ions accelerate in a constant electric field at low pressure.
At high electric field strengths the ion drift velocity is not directly proportional to the applied field and the mobility, Kh, is not a constant, but rather dependent on the applied electric field. This dependence has been exploited to develop high field asymmetric waveform ion mobility spectrometry (FAIMS) where ions are separated by a difference in their mobility at high field strength, Kh, relative to their mobility at low field strength, K. In FAIMS, ions are separated due to the dependent behavior of Kh as a function of the applied electric field strength.
A FAIMS spectrometer has an analyzer region defined by the space between two electrodes. One electrode is maintained at a selected dc voltage, often at ground potential, while the second electrode has an asymmetric waveform V(t) imposed upon it that is a repeating pattern of a short, th, high voltage, Vh, periods and longer, tl, lower voltage, Vl, periods such that Vhth+Vltl=0 for each complete cycle of the waveform. The peak voltage during the high voltage portion of the waveform is called the “dispersion voltage” or DV.
Ions to be separated are entrained in a stream of gas flowing through the FAIMS analyzer region. The net motion of an ion is the sum of an axial x-axis component due to the stream of gas and a transverse y-axis component due to the applied electric field. The distance traveled by an ion during the high voltage portion of the waveform is given by dh=KhEhth where Eh is the applied field. During the low voltage portion of the asymmetric waveform, dl=KEltl. As Ehth and Eltl are equal in magnitude the net displacement along the y-axis occurs because of the difference in Kh and K. This transverse drift is compensated by applying a constant voltage to the first electrode, the “compensation voltage” or CV. Hence, where multiple ions are present, only an ion whose drift is compensated can arrive at a detector for an appropriate combination of DV and CV. Analysis can be carried out by changing CV over time.
Buryakov et al. Int. J. Mass Spectrom. Ion Processes, 128, 143 (1993) disclosed the first FAIMS device with planar electrodes. The electric field between the planar electrodes is uniform, allowing ions to diffuse laterally. Because there is a lack of ion focusing, poor ion transmission into the narrow outlet, which is often the entrance to a mass spectrometer, affects sensitivity. The use of curved electrodes produces a two-dimensional atmospheric pressure ion focusing effect that achieves greater ion transmission efficiencies. For example, Carnahan et al. U.S. Pat. No. 5,420,424, describes a device where two cylindrical electrodes are used where one electrode is concentrically located within a tubular electrode and the ions are transmitted parallel to the central axis of the cylinders. Guevremont et al., WO 00/08455, describe a domed-FAIMS analyzer where a cylindrical inner electrode has a curved surface terminus proximate an ion outlet orifice to an analyzer region. The application of an asymmetric waveform to the inner electrode has an additional ion-focusing action that extends around the spherically shaped terminus of the inner electrode that causes the selected ions to be directed radially inwardly within the region proximate the inner electrode terminus. Guevremont et al., WO 1/69216 disclose a “side-to-side” FAIMS. In this design the ions are transmitted around the circumference of an inner cylindrical electrode. The ion inlet and the ion outlet of a side-to-side FAIMS device are disposed, one opposing the other, within a surface of the outer electrode. An ion is selectively transmitted through the curved analyzer region between the ion inlet and the ion outlet along a continuously curving ion flow path perpendicular to the central axis of the cylinders. The ions travel approximately fifty percent of the circumference of the inner electrode and are partitioned between two streams traveling in opposite directions around the inner electrode, effective reducing the ion density within the analyzer region, reducing the ion-ion repulsion space charge effect, and allowing a reduction of the travel distance to improve the ion transmission efficiency. However, in this design the ions are not focused in a direction parallel to the central axis of the cylindrical electrodes and an inner cylinder with a small radius is required to produce a strongly focused field, which can result in ion transit times that can be insufficient to separate mixtures of different ions.
Both planar and cylindrical (“dome” and “side-to-side”) geometries have been used for commercial FAIMS systems: the DMS (Sionex, Bedford, Mass.); and Selectra (Ionalytics, Ottawa, Canada). These commercial systems are used for detection of drugs, explosives, chemical warfare agents, environmental monitoring, bacterial typing, product quality assurance, natural resource management, and biomedical research. However, the use of FAIMS has often been limited by resolving power (Rp) which is about an order of magnitude less than that of the Ion Mobility Spectrometry (IMS) designs.
Shvartsburg et al., Anal Chem. 2006, 78(11), 3706-14 has compared the inherent resolving power of simulated planar and cylindrical FAIMS systems. As one proceeds from a curved surface of 8 mm with a Rp of 10 to a curved surface of 73 mm with an Rp of 45, the ion transmission suffers from 96% at Rp=10 to 2% at Rp=45. At higher curvatures, approaching ∞ for a planar surface, although Rp should increase significantly, the ion transmission would become insufficient for practical use. One critical difference between planar and cylindrical FAIMS is in the dependence of peak widths on the ion residence time. In a cylindrical system, ions with CVs outside of a finite range are filtered out, allowing equilibrium in the gap that requires a certain residence time (˜50 ms for the commercial Selectra), and greater FAIMS resonance time does not improve resolution. In planar systems a single CV permits ion equilibrium and ions with even a small CV difference will eventually be eliminated with sufficient time. Hence, longer separation times would increase Rp (in principle) indefinitely. However, longer residence time can further diminishes the ion transmission due to lateral diffusion. Commercial planar systems typically have a residence time that is about two orders of magnitude less than that of a cylindrical system to have similar ion transmission. Tang et al., U.S. Patent Application Publication 2007/020059 discloses the use of an “ionic funnel” at a non circular ion outlet of a planar FAIMS unit where an ion transmission of more than two fold was displayed because virtually no loss is experienced between the outlet from the FAIMS ion outlet and the outlet of the “ionic funnel.”
Hence, it would be advantages to use FAIMS units that can increase the resolving power if better or additional means of diminishing the effects of lateral diffusion of the ions in the analyzer region could be achieved to improve ion transmission.